The present invention relates to the operation of BLDC motors and in particular to the operation of BLDC motors at start up and at low to medium speeds.
Driving of BLDC motors requires the knowledge of the rotor position relative to the stator coils. For a given rotor position, a current is forced through the stator coils in the appropriate direction to generate torque so as to turn the rotor in a desired direction. When the rotor has turned beyond a certain position, (the commutation point), the current direction needs to be varied (commutated) so that it is again in the appropriate direction to generate torque in the desired direction.
In some implementations, the rotor position information can be determined using sensors, such as Hall-effect sensors or by appropriate means. In other implementations, the BEMF voltage induced in the stator coils by the rotor can be monitored to determine rotor position. These methods work best when the motor is operating at a steady speed above a particular threshold speed. Rotor position detection by these methods is thus less reliable at slow speeds and at start up or in conditions where the load on the motor varies over a wide range. For example, this may occur in motors used to drive oil pumps. Due to the strong dependency of oil viscosity on temperature, the required starting torque can vary by a factor 100 or more. Other examples include motors used in applications such as pumps, fans or actuators where operation at near-zero-speed is required.
These issues have been addressed in IEEE Industry Applications Society, Annual Meeting, New Orleans, La., Oct. 5-9, 1997; “Initial Rotor Angle Detection Of A Non-Salient Pole Permanent Magnet Synchronous Machine,” Peter B. Schmidt, Michael L. Gasperi, Glen Ray, Ajith H. Wijenayake; Rockwell Automation wherein six current pulses are imposed on the driving waveform of a motor and the current rise times are evaluated to determine rotor position. The problems are also addressed in U.S. Pat. No. 6,885,163, wherein only two test pulses are used, timed to occur when the rotor position is known to be within a range of 60° of a particular estimated position. Both of these methods have the draw back that a currentless waiting time is needed before starting the next test pulse in order to accurately compare the measured rise times. During this currentless waiting time, the torque producing current is interrupted and hence it is not possible to deliver a maximum torque. Additionally, the interruption of the torque can lead to unstable start-up, especially under heavy load conditions.
An alternative way of addressing this problem is disclosed in U.S. Pat. No. 5,796,235. In this scheme, test pulses are imposed at intervals and the current change is measured differentially at the beginning and the end of each test pulse. These dI/dt values are then evaluated by a complex trigonomic function to calculate the current rotor position. Accordingly, it does not suffer from the drawback that the test pulse currents must have decayed before the next pulse can be initiated. However, the position is only measured when test pulses are inserted and insertion of test pulses also distorts the torque.